A photodiode is a diode that generates electric current upon receiving light. Photodiodes widely used as a light-receiving element for Charge Coupled Device (CCD) sensors, CMOS sensors, or other solid-state sensor. A photodiode is made up of pn junction semiconductors. When the pn junction is reverse-biased and a high electric field is applied, the depletion layer widens. Incident light is absorbed mainly in the depletion layer and generates electron-hole pairs. The obtained charge is held by the potential generated in the photodiode and is detected as current or voltage. In a solid-state sensors, pixels having diodes with this configuration are arranged in an array.
When a pixel is irradiated with high intensity light, the charge generated by this irradiation will overflow from the pixel to adjacent pixels if the photocharge exceeds the charge storage capacity of the photodiode. This phenomenon is known as blooming. For solid-state sensors, overflow drains that drain the excess photocharge that had saturated a pixel are widely used as an anti-blooming structures. Overflow drains include lateral overflow drains or LOD that drain the excess photocharge in the in-plane direction of the principal surface of a substrate and vertical overflow drains or VOD that discharge the photocharge in the direction perpendicular to the principal surface of the substrate and toward the substrate.
Turning to FIG. 1A, an example of a conventional solid-state sensor 100 having LODs can be seen. Sensor 100 generally comprises pixels 102-1, 102-2, and 102-3 arranged in a row. Each of these pixels 102-1, 102-2, and 102-3 includes photodiodes 110-1 and 110-2-1, 110-1 and 110-2-2, and 110-1 and 110-2-3 that are separated by pixel separating regions 106-1 and 106-2. Within each of the barrier regions 106-1 and 106-2 are (respectively) n-type regions 108-1 and 108-2 and p-type regions 110-1 and 110-2 that operate as LODs.
As can be seen in FIG. 1B (which is a cross-sectional view along A-A of sensor 100), sensor is comprised of a layered semiconductor materials. Generally, an n-type region or layer 114 and p-type region or layer 116 are formed on or over p-type substrate 112 to form a pn junction that constitutes photodiodes 110-1 and 110-2-1, 110-1 and 110-2-2, and 110-1 and 110-2-3 that are adjacent to one another. The barrier regions 106-1 and 106-2 are formed between adjacent photodiodes 110-1 and 110-2-1, 110-1 and 110-2-2, and 110-1 and 110-2-3, each with an LOD having an n-type region 108-1 and 108-2 bordered by a p-type region 110-1 and 110-2 formed along the perimeter.
Turning FIG. 1C is a potential diagram for the cross-sectional view of FIG. 1B is shown. As shown, a potential well is formed between the adjacent photodiodes 110-1 and 110-2-1, 110-1 and 110-2-2, and 110-1 and 110-2-3, with the peaks P1 and P2 begin located respectively at the junctions between the photodiodes 110-1 and 110-2-1, 110-1 and 110-2-2, and 110-1 and 110-2-3 and the barrier regions 106-1 and 106-2 and at the center of the barrier regions 106-1 and 106-2. Thus, photocharge (photoelectrons) can be stored in photodiodes 104-1, 104-2, and 104-3.
In operation, a predetermined voltage can be applied to the n-type regions 108-1 and 108-2. As indicated by potential P2 in FIG. 1C, the potential of the n-type regions 108-1 and 108-2 are shifted to the high potential side. At the same time, the barrier of the potential of the p-type regions 110-1 and 110-2 are modulated to the high potential side. At that time, when the charge C stored in the potential valley on the photodiodes 104-1, 104-2, and 104-3 sides overflows, the charge will pass the potential barrier of the p-type region 110-1 and 110-2 part to drain into n-type region 108-1 and 108-2.
The LOD with this configuration is used to drain the charge. It is insensitive to the storage of charge or light. Consequently, in order to increase the sensitivity of the photodiode, and hence the sensitivity of the solid-state sensor, it is desired to form it with a layout that is as small as possible. However, miniaturization is difficult since p-type regions 110-1 and 110-2 act as the barriers formed around n-type regions 108-1 and 108-2, which act as the drain. Consequently, the width of the LOD region becomes larger than the width of barrier regions 106-1 and 106-2 as shown in FIG. 1A, which leads to a decrease in the sensitivity of the photodiode and solid-state sensor and a decrease in storage capacity.
Therefore, there is a need for an improved solid-state sensor.